Vehicle suspension system

ABSTRACT

A vehicle suspension system incorporating suspension units (46-48) associated with respective vehicle wheels and selectively controllable to vary the ride height of the vehicle. Vehicle speed sensors and height sensors are responsive to departure of the ride height from a predetermined height datum which is a function of the vehicle speed and to provide a signal related thereto to control movement of at least one suspension unit thereby to tend to move at least a part of the vehicle to said predetermined height datum. Timer means are provided so that the movement of a suspension unit in response to a signal from a height sensor is inhibited until a signal of at least a predetermined magnitude has persisted for at least a prescribed part of a prescribed period of time.

BACKGROUND OF THE INVENTION

This invention relates to a vehicle suspension and in particular,although not exclusively, to a suspension for an untracked wheeledvehicle. It relates also to a vehicle incorporating the suspension ofthe invention and to method of controlling a vehicle suspension.

The invention is directed particularly but not exclusively to a vehicleof a kind having an air or like gas suspension.

The invention seeks to provide a suspension which offers a facility toachieve good ride characteristics related to the operating condition ofthe vehicle.

SUMMARY OF THE INVENTION

In accordance with one of its aspects the present invention provides avehicle suspension comprising in. combination vehicle speed sensingmeans, suspension units associated with respective vehicle wheels andselectively controllable to vary the ride height of the vehicle, eachsuspension unit incorporating an electrical height sensing deviceresponsive to departure of the ride height from a predetermined heightdatum which is a function of the vehicle speed and to provide a signalrelated thereto to control movement of at least one suspension unitthereby to tend to move at least a part of the vehicle to saidpredetermined height datum and timer means whereby the movement of asuspension unit in response to a signal from a height sensing device isinhibited until a signal of at least a prescribed magnitude haspersisted for at least a prescribed part of a prescribed period of time.

The suspension units each may be gas, e.g pneumatic suspension units andvalve means responsive to signals from the height sensing device may beprovided for selective control of gas to or from each suspension unit.

Each suspension unit may incorporate a damping device and the dampingdevice may be of a kind which incorporates an electrical height sensingdevice to provide a signal related to departure of the ride height froma predetermined height datum. An example of a suitable damper unit is alinear variable differential transformer type unit such as described inthe specification of UK Patent Application GB 2027207A. Alternativelyelectrical height sensing devices such as of the types comprisingultrasonic displacement sensors or potentiometers may be employed.

The invention further provides a variable sensitivity system in whichthe period for which the timer means inhibits movement of a suspensionunit in response to a signal from a height sensor is a function of themagnitude of that signal. The sensitivity variation may be substantiallycontinuously variable whereby for example the time delay is a functionof an averaged value of the signal from the height sensor or it may beof a stepped kind in which when the magnitude of the signal exceeds aprescribed magnitude there is no time delay or only a short period oftime delay whereas when the magnitude of the height departure signal isbelow a prescribed magnitude the timer means causes a delay of at leasta prescribed magnitude.

It is envisaged that the system may have two sensitivity modes, whichmay be known as insensitive and sensitive modes respectively. Thesensitive mode may be invoked for example during initial start up of thevehicle to result in speedy supply of gas to gas suspension unitsthereby to place the vehicle at approximately the desired ride height.That initial desired ride height may be a height lying within aprescribed band. After a prescribed period following initial start upthe system may change to an insensitive mode of operation in which theride height information is used to position the vehicle more accuratelyto a particular height. The later positioning may be achieved by thesignals from the height sensors indicating departure from a prescribeddesired height; if that signal persists continuously for at least aspecified period of time or at least a specified proportion of aprescribed period of time, that signal may be regarded as indicative ofa valid error situation and control means associated with the suspensionsystem may operate to adjust the vehicle height by admitting orexhausting gas from one or more of the suspension units.

More particularly, the vehicle suspension may comprise an electroniccontrol unit having an algorithm adapted to collate and store relevantinput data as a vehicle traverses a road and to eliminate irrelevantinput data thereby to ensure that the system does not respondinappropriately to transient inputs. In one example the algorithm may beadapted to decide which of the four height sensors has the largest errorrelative to a datum, to decide whether the error is above or below thedatum and whether the error is outside pre-set limits. If the error isgreater than the pre-set limits a timer may be initiated to set a firsttime delay T1. If the direction of error should change during the timeT1, the timer may be adapted to be re-set. However, if the timer timesout, i.e. there is no intermediate change of direction, then theelectronic control unit may cause gas to be admitted or exhausted fromone or more of the suspension units.

The magnitude of T1 may have two values dependent upon whether thesystem is to be operating in a sensitive or an insensitive mode. Theselection of mode may be achieved automatically having regard to thevehicle speed and/or for example to the magnitude of height error from aprescribed datum. The system may be arranged to select the sensitive(i.e. rapid response) mode automatically when a change of ride heightstate of at least a certain magnitude has been demanded. The demandmight arise automatically having regard to information from the heightsensor means or as a result of a manual information input to theelectronic control unit (e.g when changing from a manual height overridesituation to an automatic normal mode of operation). The insensitivemode may be invoked automatically by the vehicle attaining a heightlying within a prescribed error band from the desired ride height. Inthat case the timer T1 has a value much greater than when in thesensitive mode. The purpose of this greater time is to ensure that thesystem does not respond inappropriately to transient road input data.Thus operation of control equipment to admit or exhaust gas from thesuspension units is inhibited unless reasonably necessary, and energy tomaintain a supply of compressed gas is conserved.

The vehicle suspension may have associated therewith an electroniccontrol unit which operates automatically to control the suspensionunits in response to received information and which also operates inresponse to a manual override. The system may be adapted to facilitatemanual override for example to put the suspension in a kneel conditionin which the vehicle is lowered; the electronic control unit may beprogrammed to allow a kneel condition to be achieved only if the vehicleis stationary and/or the hand (park) brake is applied to resist vehiclemovement.

The manual override may be operable to a high profile condition in whichthe ride height is selected by the driver to be greater than normallywould prevail in automatic operation of the system, The electroniccontrol unit may be arranged to allow entry to the high profile modeonly if the vehicle speed is below a prescribed threshhold, e.g 35 milesper hour, and it may furthermore be arranged to revert automatically toa standard ride height mode in the event of the vehicle speed exceedinga prescribed figure, which may be aforementioned speed which must not beexceeded for entry into the high profile mode, or another datum speed.

The system may also incorporate a manual override which locks thesuspension to a standard ride height in the case of a vehicle used fortowing. Alternatively, sensor means may be provided automatically todetect when the vehicle is coupled to a load for towing andautomatically to inhibit significant departures from a standard rideheight, e.g to a high or low ride height at least at such times as thevehicle is in motion.

The electronic control unit may be adapted automatically to lower theride height of the vehicle from a standard to a lower height when thevehicle speed exceeds a prescribed magnitude, e.g 60 miles per hour. Thecontrol unit may be adapted also to return to a normal ride height ifthe vehicle speed falls below a prescribed figure for at least aprescribed period and/or to revert forthwith to a normal, standard modeif there is a significant reduction of speed to below another prescribedspeed.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of an electronically controlled vehicle suspension system inaccordance with the present invention will now be described withreference to:

FIG. 1 which is a plan view of part of a system in accordance with thepresent invention;

FIG. 2 which is a section view of a damper/height sensor unit;

FIG. 3 which is a schematic of a spring control system; and

FIG. 4 which shows examples of typical system parameters,

An example of valve means for use in the present invention are describedwith reference to the accompanying diagrammatic drawings in which:

FIG. 5 which shows schematically a pneumatic suspension unit inaccordance with the present invention;

FIG. 6 shows a modified form of the embodiment of FIG. 5, and

FIG. 7 is a cross-sectional view of a second, cushion exhaust valve ofthe suspension of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following table lists major components of the system.

The electronically controlled air suspension system as fitted to a fourwheel, four wheel drive vehicle comprises the following majorcomponents.

Front Air Spring Assembly

Rear Air Spring Assembly

Electronic Control Unit

Reservoir Assembly

Air Harness

Mounting Bracket and Ancilliary Equipment, comprising:

(i) Air Supply System comprising:

Mounting Brackets

Compressor Assembly

Air Dryer Assembly

Isolation Mounts

(ii) Assembly 10--Valve Block & Air (see FIG. 1) Service Unitcomprising:

Solenoid Operated Air Distribution Valve Block 11

Air Service Unit 12 incorporating:

Non return Valve

Pressure Switch

Filter

Schrader Connection

Air Pipe Connector

Damper--Height Sensor Assembly (see FIG. 2 )

Driver Operated Functional Switches

The suspension spring media is provided by four air springs whichreplace the conventional coil springs.

The air springs provide a variable rate spring which achieve nearconstant ride frequency for all load conditions. Thus a significantimprovement in overall ride is attainable.

The high pressure (13 bar) compressed air is provided by a 12V D.C.wobble piston Compressor which incorporates life of the vehicle brushes,radio frequency interference suppression and thermal trip protection.

To reduce air induction noise and protect the compressor cylinder bore,a serviceable air intake silencer/filter is fitted.

The compressed air passes through a oneway valve (incorporated into theair service unit), to a 5L Reservoir which stores air at a nominal 13bar pressure (see FIG. 3).

The reservoir, in addition, acts as a drop tank for the moisture thatthe compressed air will contain. Consequently, the reservoir must bedrained periodically using the drain plug provided.

The compressed air is taken from the reservoir to the Air Service Unitwhich has a replaceble main air filter and a pressure switch. Thepurpose of the Pressure Switch is to maintain the system pressurebetween set limits by switching the compressor on and off via an E.C.U.controlled relay.

Air is then passed to the solenoid (six) operated air distribution valveblock. The purpose of the valve block is to control the flow of air to,and from, the four air springs dependent upon system demands asdetermined by the inputs to the E.C.U..

Air flow to, and from, the air springs is controlled via pilot (air)operated high flow diaphragm valves. The pilot air is controlled by thesix solenoid operated valves.

The pilot air line incorporates non-return valves in order to preventloss of pilot pressure, and therefore air spring pressure, if there is atotal loss of reservoir pressure.

For air to be admitted to any air spring the main air feed solenoidvalve must be energized in addition to the relevant air spring solenoidor solenoids.

Conversely, for air to be exhausted from any spring the exhaust solenoidvalve must be energized in addition to the relevant air spring solenoidor solenoids.

A silencer is fitted to the exhaust port of the valve block.

Air is passed through a dryer prior to being fed through the high flowdiaphragm valves and into the air springs.

Conversely, air exhausted from the air springs passes through the dryerin the reverse direction prior to exhaust to atmosphere via the exhaustvalve.

Moisture is removed from the air as it passes vertically upwards throughthe dryer dessicant. The dessicant in the lower portion of the dryerbecomes `wet`.

During exhaust the dry air from the air springs passes verticallydownwards through the `wet` dessicant absorbing moisture prior toventing to atmosphere.

This purging action regenerates the air dryer.

As stated, the purpose of the system is to provide four height sensormodes. The height of the vehicle, relative to the axles, is provided bysensors incorporated in each damper.

A variable inductive height sensor is incorporated in the dust cover ofeach damper 30 (see FIG. 2) and provides a signal whose level isdependent upon damper displacement.

The suspension is controlled by an eight bit microprocessor ElectronicControl Unit which operates in one of the several states shown belowstatus chart. During each state the E.C.U. maintains the requested rideheight by adjusting the volume of air in an air spring attached to eachwheel.

The controller determines engine rotation by measurement of the periodof a phase of the vehicle alternator. If this period is greater than arequired value the engine shall be considered stopped and all suspensionfunctions will be inhibited except that level vehicle on parking mode.

This is to prevent the compressor drawing a large current from thebattery when the alternator is not charging.

The controller calculates a value of vehicle speed by measuring theperiod between pulses from a speed sensor. The value is used todetermine an allowable transition from one suspension state to another.

The controller adjusts the height of each suspension unit in accordancewith the demanded state.

To raise the height the appropriate air spring valves are selectedtogether with the main air feed inlet valve. To lower the height theappropriate air spring valves are selected with the exhaust valve.

When raising the height, the rear of the vehicle shall be raised firstby approximately 70% of the required height change followed by raisingthe front to 70% of the required change. The remaining 30% change willbe achieved by individual adjustment of each suspension unit.

Lowering of the vehicle will be complimentary to raising, with the frontof the vehicle being lowered first.

This will ensure that when the headlamps are illuminated there is noinconvenience to other road users.

However, when lowering to the kneel position all air valves are openedat the same time to achieve a fast response.

The E.C.U. accepts switch inputs for the following functions:

Handbrake

Footbrake

Door Switch (open/closed) except tailgate

Reservoir pressure

Up

Down

Inhibit

The handbrake switch is used to control the kneel demand. Kneel cannotbe entered unless: vehicle speed is zero, all doors closed, enginerunning and HANDBRAKE APPLIED.

Door (except tailgate) switch (open/closed) is used to control kneeldemand. Kneel cannot be entered unless: vehicle speed is zero, ALL DOORSCLOSED, engine running and handbrake applied.

When the FOOTBRAKE is on, and for a period of one second after it isoff, all height levelling is suspended.

The purpose of this inhibit is to prevent the system reacting totransient suspension movement caused by weight transfer during braking.

This inhibit function is removed after a period of 80 seconds regardlessof footbrake state.

When the E.C.U. detects an output from the PRESSURE SWITCH indicatinglow pressure, then the E.C.U. operates the pump relay until the pressureswitch indicates normal pressure. The pump relay will not operate unlessthe engine speed is greater than 500 r.p.m..

The HIGH PROFILE ride state is driver selected by depressing themomentary UP switch. High profile raises the vehicle body byapproximately 45 mm at road speeds below 40 m.p.h.. If road speedexceeds this figure the E.C.U. automatically reverts to standard rideheight. High profile then has to be re-selected, if required, and isdependent upon conditional requirements (see FIG. 4).

Lowering the vehicle from high profile to standard ride height isachieved by depressing the momentary down switch twice.

The above are dependent upon fulfilment of all conditional requirements.

The self latching inhibit switch is engaged to maintain the suspensionat the standard ride height. That is, automatic height adjustment andthe up and down momentary switches are inoperative (inhibited). Thisshould be engaged when towing.

Note that engaging the inhibit switch will automatically return thevehicle to standard ride height from any other height mode.

General freeze state is entered at any time on the E.C.U. detecting apassenger door opening. All height control is suspended in this statewith all height control valves closed.

The E.C.U. will maintain the system in this state until the criterionfor entering any state are met.

The freeze state is intended as a SAFETY factor and thus should not beartificially overriden.

A levelling mode on parking is incorporated into the E.C.U.functionality wherein the vehicle will continue to level to the lowestcorner, for approximately 10 seconds, after the vehicle is exited andall doors closed.

The Electronic Control Unit incorporates Fault Recovery Strategies tominimise the effect of a sensor, or actuator failure.

A serial data link is provided to allow diagnostics information to bedisplayed and to set height sensor datums at the end of vehicle build orservice.

The valve means of FIG. 5 comprises a valve manifold unit 40 having acentral manifold chamber and four direct acting electrically operatedsolenoid valves 42-45 selectively controllable to allow pressurised airto be admitted to a respective air spring 46-49 or exhausted therefrom.

The manifold also incorporates three other electrically operatedsolenoid valves 50-52. One valve is a supply valve to allow pressurisedair to be admitted to the chamber 41 from an external reservoir via anon-return valve 54. Another valve 51 is a first exhaust valve andinterconnects with a second exhaust valve 52 via a first common flowpath 55, an externally mounted regenerative drier 56 and a second commonflow path 57.

An externally located compressor 58 connects via passage 58' with thesecond common flow path 57. The valve manifold also incorporates anexhaust passage 59 from the valve 52, an auxiliary supply passage 60having a one-way valve 61 and interconnecting the reservoir 53 with asupply passage part 62 of the first common flow passage 55, and a nonreturn valve 63 in that other part of the passage 55 between the supplypart 62 and the first exhaust valve 51.

The valve means also incorporates other conventional items such as asafety relief valve 64, pressure switch 65 to initiate operation of thecompressor 58 and air inlet filter 66.

In the aforedescribed apparatus the pressure in chamber 41 is controlledby operation of the valves 50,51 and is selected in accordance with theinstantaneous requirements of a particular spring 46-49 which is put incommunication with the chamber via 49 a respective valve 42-45.

When the reservoir pressure falls below a predetermined level and valves51 and 52 are not in an exhaust mode the compressor 58 supplies filteredair to the reservoir 53 via the second common flow 57, drier 56, supplypassage 62, non-return valve 61 and line 60.

When it is required to exhaust the chamber 41 any operation of thecompressor is inhibited and the exhaust valves 51,52 are opened to allowair to flow via passage 55 and non-return valve 63 to passregeneratively through drier 56 and then passage 57 and valve 52 to theexhaust line 59.

The embodiment of FIG. 5 may be modified by replacing the second exhaustvalve 52 with a pilot operated type valve 70 as shown schematically inFIG. 6, and the valve as shown in cross-section in FIG. 7. Parts commonwith FIG. 5 bear corresponding reference numerals.

The valve 70 has a main exhaust path via passage 71, which leads frompassage 57 and over a first face of a valve diaphragm 72 to an exhaustplenum 73 connected to the exhaust line 59.

The valve diaphragm 72 is biased closed against passage 71 by acompression spring 74 and its face opposite the first face is acted onalso by pressure of gas in the pilot chamber 75. The pressure in thepilot chamber is under the control of an electrically actuated solenoidvalve plunger 76. When plunger 76 is against a first valve seat 77 asshown in FIG. 7 the second valve seat 78 is open and a pilot exhaustline 79 results in the pilot chamber 75 being at atmospheric pressure.When the plunger 76 lies against the seat 78 the chamber is exposed tothe pressure in the first common flow path 62 with which itinterconnects via a pilot feed line 80.

In use of the valve of FIG. 7, as incorporated in the valve means ofFIG. 5 in place of the second exhaust valve 52, during normal exhaustmodes the plunger 76 is seated against seat 77 so that exhaust occursonly via the drier and passage 71, thereby obtaining regenerative dryingof the drier by all of the exhausting gas. In this position chamber 75is open to atmosphere and gas exhausting via passage 71 is able tocounter the effect of bias spring 74 and thereby lift the diaphragm fromthe seat of passage 71.

In the case of the compressor being called upon to supply pressurizedair to the reservoir the solenoid of valve plunger 76 is operated tolift the plunger from seat 77 to bear against seat 78, thus closingchamber 75 from the pilot exhaust line. The pressure in chamber 75 willthus be that in line 62 which is at or substantially equal toatmospheric pressure because of the presence of the reservoir's one-wayvalve 61 and the facility for line 62 to exhaust via passsage 71 whenthe compressor is not operational. Hence on initial start up of thecompressor 58 the pressure in line 57 is at or substantially equal toatmospheric pressure. The compressor is not required to suffer thestrain and wear associated with start up against a significant pressurehead.

Upon initial start up some gas will tend to exhaust via passage 71against the bias of spring 74, but progressive increase of pressure inline 62 will act via line 80 and chamber 75 to cause the diaphragm 72 tobe closed, it being noted that the area of diaphragm exposed to thepressure in chamber 75 is much greater than that area within the valveseat at the end of passage 71. Thus after a momentary initial start upperiod the line 71 is closed by action of the pressure built up inchamber 75 and the reservoir can be recharged.

I claim:
 1. A vehicle suspension of variable sensitivity comprising in combination vehicle speed sensing means,suspension units associated with respective vehicle wheels and selectively controllable to vary the ride height of the vehicle, each suspension unit incorporating an electrical height sensing device responsive to departure of the ride height from a predetermined height datum which is a function of the vehicle speed and to provide a signal related thereto to control movement of at least one suspension unit, thereby to tend to move at least a part of the vehicle to said predetermined height datum and timer means whereby the movement of a suspension unit in response to a signal from a height sensing device is inhibited as a function of the magnitude of until a signal of at least a predetermined magnitude has persisted for at least a prescribed part of a prescribed period of time.
 2. A vehicle suspension according to claim 1 in which said suspension units are gas suspension units.
 3. A vehicle suspension according to claim 1 comprising valve means responsive to signals from the height sensing device for selective control of gas to and from each suspension unit.
 4. A vehicle which incorporates a suspension according to claim
 1. 5. A vehicle suspension according to claim 1 in which the sensitivity variation is substantially continuously variable.
 6. A vehicle suspension according to claim 1 in which the sensitivity variation varies in a stepped manner.
 7. A vehicle suspension according to claim 1 having a sensitive mode during which the vehicle speedily adopts approximately the desired ride height and an insensitive mode in which the vehicle is more accurately positioned to a prescribed ride height.
 8. A vehicle suspension according to claim 1 comprising an electronic control unit to receive information from the height sensing device and having associated therewith timer means whereby the timer means initiates a timed period if a height error of greater than a preset limit occurs, the electronic control unit being arranged to cause a height adjustment of the vehicle if the height error persists for greater than the prescribed period of time to reset the timer means in the event of the direction of the height error changing during the initially set period of time.
 9. A vehicle suspension according to claim 7 in which the timed period has two values dependent on whether operating in accordance with the sensitive or insensitive.
 10. A vehicle suspension according to claim 1 including a manual override operable to change the preselected ride height.
 11. A vehicle suspension according to claim 10 in which the vehicle suspension incorporates an electronic control unit arranged to inhibit operation of the manual override depending on the speed of the vehicle.
 12. A vehicle suspension according to claim 10 in which the system incorporates an electronic control unit arranged to revert to automatic height control from a manual override condition in the event of change of the vehicle speed to a prescribed speed or speed range.
 13. A vehicle suspension according to claim 11 characterised in that the electronic control unit is responsive to information indicating that the vehicle is coupled to a load for towing and does not counter a manual overide instruction to lock the suspension to a standard ride height when towing.
 14. A vehicle suspension comprising in combinationvehicle speed sensing means, suspension units associated with respective vehicle wheels and selectively controllable to vary the ride height of the vehicle, each suspension unit incorporating an electrical height sensing device responsive to departure of the ride height from a predetermined height datum which is a function of the vehicle speed and to provide a signal related thereto to control movement of at least one suspension unit, thereby to tend to move at least a part of the vehicle to said predetermined height datum, a manual override to change the preselected ride height comprising an electronic unit responsive to information indicating that the vehicle is coupled to a load for towing and does not counter a manual override instruction to lock the suspension to a standard ride height when towing; timer means whereby the movement of a suspension unit in response to a signal from a height sensing device is inhibited until a signal of at least a predetermined magnitude has persisted for at least a prescribed part of a prescribed period of time. 